On Wed, Jun 24, 2020 at 10:59:47AM -0700, Stefano Stabellini wrote: > On Wed, 24 Jun 2020, Michael S. Tsirkin wrote: > > On Wed, Jun 24, 2020 at 05:17:32PM +0800, Peng Fan wrote: > > > Export xen_swiotlb for all platforms using xen swiotlb > > > > > > Use xen_swiotlb to determine when vring should use dma APIs to map the > > > ring: when xen_swiotlb is enabled the dma API is required. When it is > > > disabled, it is not required. > > > > > > Signed-off-by: Peng Fan <peng.fan@xxxxxxx> > > > > Isn't there some way to use VIRTIO_F_IOMMU_PLATFORM for this? > > Xen was there first, but everyone else is using that now. > > Unfortunately it is complicated and it is not related to > VIRTIO_F_IOMMU_PLATFORM :-( > > > The Xen subsystem in Linux uses dma_ops via swiotlb_xen to translate > foreign mappings (memory coming from other VMs) to physical addresses. > On x86, it also uses dma_ops to translate Linux's idea of a physical > address into a real physical address (this is unneeded on ARM.) > > > So regardless of VIRTIO_F_IOMMU_PLATFORM, dma_ops should be used on Xen/x86 > always and on Xen/ARM if Linux is Dom0 (because it has foreign > mappings.) That is why we have the if (xen_domain) return true; in > vring_use_dma_api. VIRTIO_F_IOMMU_PLATFORM makes guest always use DMA ops. Xen hack predates VIRTIO_F_IOMMU_PLATFORM so it *also* forces DMA ops even if VIRTIO_F_IOMMU_PLATFORM is clear. Unfortunately as a result Xen never got around to properly setting VIRTIO_F_IOMMU_PLATFORM. I would like to make Xen do what everyone else is doing which is just set VIRTIO_F_IOMMU_PLATFORM and then put platform specific hacks inside DMA API. Then we can think about deprecating the Xen hack in a distance future, or hiding it behind a static branch, or something like this. > You might have noticed that I missed one possible case above: Xen/ARM > DomU :-) > > Xen/ARM domUs don't need swiotlb_xen, it is not even initialized. So if > (xen_domain) return true; would give the wrong answer in that case. > Linux would end up calling the "normal" dma_ops, not swiotlb-xen, and > the "normal" dma_ops fail. > > > The solution I suggested was to make the check in vring_use_dma_api more > flexible by returning true if the swiotlb_xen is supposed to be used, > not in general for all Xen domains, because that is what the check was > really meant to do. Why not fix DMA ops so they DTRT (nop) on Xen/ARM DomU? What is wrong with that? > > In this regards I have two more comments: > > - the comment on top of the check in vring_use_dma_api is still valid > - the patch looks broken on x86: it should always return true, but it > returns false instead > > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > > > index a2de775801af..768afd79f67a 100644 > > > --- a/drivers/virtio/virtio_ring.c > > > +++ b/drivers/virtio/virtio_ring.c > > > @@ -252,7 +252,7 @@ static bool vring_use_dma_api(struct virtio_device *vdev) > > > * the DMA API if we're a Xen guest, which at least allows > > > * all of the sensible Xen configurations to work correctly. > > > */ > > > - if (xen_domain()) > > > + if (xen_vring_use_dma()) > > > return true; > > > > > > return false; > > > > > > The comment above this should probably be fixed. > > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
